1. Technical Field of the Invention
The present invention relates to a structural member having a closed section, and an apparatus and a method for producing the structural member. More particularly, the invention relates to a structural member formed of a plate-like work and having a channel-shaped sectional portion and two closed sectional portions for which both side walls of the channel-shaped sectional portion are commonly shared, and an apparatus and a method for producing a structural member having such a closed section.
2. Description of Related Art
A bumper reinforcement which constitutes an automobile bumper has been known as this type of the structural member. As has been disclosed in Japanese Patent Publication Laid-Open No. Hei 10-278706, the bumper reinforcement is attached and fixed by inserting the tip end portion of the side members of an automobile into the channel-shaped sectional portion, and this is bent or curved in compliance with the shape of a bumper which constitutes a part of the design of the automobile.
Referring to FIG. 17, the channel-shaped sectional portion 51 of a bumper reinforcement is defined by the bottom wall 60 formed substantially at the middle of a plate-shaped work Wxe2x80x2 (FIG. 11), and boundary walls (side walls) 61 each forming a boundary to a closed sectional portion 52 formed by bending the plate-shaped work from the respective ends of the bottom wall 60. Also, the closed sectional portion 52 is defined by the boundary walls 61, a longitudinal inner side wall 62 located at the automobile side when the plate-shaped work is bent from the respective ends of the boundary wall 61 and attached to and fixed at a side member, a vertical outer side wall 63 located outside the automobile in the vertical direction when the plate-shaped work is bent from the longitudinal inner side wall 62 and attached to and fixed at the side member, and a longitudinal outer side wall 65 located at the opposite side of the automobile when being bent from the vertical outer side wall 63, causing the side edges of the work to overlap on the bottom wall 60 of the channel-shaped sectional portion 51 and being attached to and fixed at the side member. The respective sidewalls 60, 61, 62, 63, and 65 are formed to be planar, and the walls adjacent to each other are bent to be approximately perpendicular to each other Further, the names of these bottom wall 60, boundary wall 61, longitudinal inner side wall 62, vertical outer side wall 63, and longitudinal outer side wall 65 are used as only reference in order to describe a structural member having a closed section in the invention, but they do not intend to limit the patterns in use. It is noted that the longitudinal direction, vertical direction or the inside and outside positions may differ in compliance with actual applications.
When producing such a bumper reinforcement, generally, as shown in FIG. 11 through FIG. 18, plate-shaped work Wxe2x80x2 is prepared (FIG. 11), and the position which falls between the boundary wall 61 and the longitudinal inner side wall 62 of the work Wxe2x80x2 is bent to form so as to secure the shape of work W11 (FIG. 12). Also, the position which falls between the bottom wall 60 of the work W11 and the boundary wall 61 is bent to form it to secure the shape of the work W12 in which the channel-shaped sectional portion 51 is formed (FIG. 13), and the position which falls between the longitudinal inner side wall 62 and vertical outer side wall 63 of the work W12 is bent into a form so as to secure the shape of work W13 (FIG. 14). Next, the position which falls between the vertical outer side wall 63 and the longitudinal outer side wall 65 is slightly bent into a form so as to secure the shape of work W14 (FIG. 15). Thereafter, the position which falls between the vertical outer side wall 63 and the longitudinal outer side wall 65 is completely bent so that the side edges of the work W14 are caused to overlap on the bottom wall 60 of the channel-shaped sectional portion, thereby securing the shape of work W15 (FIG. 16) Further, a process of forming the work W11 through W15 shaped as shown in FIG. 12 through FIG. 16 from the plate-shaped work Wxe2x80x2 is carried out by press-forming so that the respective bent portions form almost perpendicular ridge lines. The width of the plate-shaped work Wxe2x80x2 is set so as to form adequate intervals so that end faces Wa, Wa at both side edges which are caused to overlap on the bottom wall 60 of the channel-shaped sectional portion 51 are not buckled by being brought into contact with each other. The work W15 bent as shown in FIG. 16 is finally curved or bent along its length in compliance with the design of an automobile bumper and is formed to the shape of work W16 (see FIG. 18). However, in order to evade various problems such as warping, undulating, or opening of the side edges of the work W15, which may occur in the lengthwise bending or curving process, after the work W11 through W15 is bent so as to become straight or linear in its lengthwise direction in the respective steps shown in FIG. 12 through FIG. 16, the side edges of the work W15 are welded to the bottom wall of the channel-shaped sectional portion where the side edges of the work W15 are caused to overlap each other (FIG. 17). In the final process of a series of steps, the structural member is generally bent, along its length at a fixed curvature radius R1, by press-forming from a shape shown with the chain line in FIG. 18 to a shape shown with a solid line therein, whereby the structural member is formed to the shape of work W16.
On the other hand, Japanese Patent Publication Laid-Open No. Sho 59-39422 discloses a method for producing a box-shaped structural member constructed so that a plate-shaped material which is shaped like a channel by a bottom wall and both side walls is set in a channel-shaped sectional portion of a die, both side walls are folded and bent while applying an axial force thereto by a rotating bending punch, the end faces of the free end portions are opposed to each other, and the tip end faces opposed to each other are welded to each other.
That is, in the invention described above, a production method has been researched and developed, in which both end faces of the free end portions are run into each other at the middle point in the width direction by folding a plate member, an upper die and a lower die are used to process as described above, wherein since a material folded to be like a channel in advance is folded and bent, the angle of the free end portions become inevitably large at the corner portion, and the free end portions may fall down inside. That is, the tip end faces of the free end portions are run into each other in the process of formation, wherein the free end portions may become horizontal before they are completely worked, or if the free end portions are pressed to the end, they are caused to fall down inside. That is, it is difficult to fold and bend the work to a desired shape, and no actual application has been brought about. This is described therein (from page 1, 5th line in the right lower column, of the Publication to Page 2, 3rd line in the right upper column thereof).
Therefore, the aforementioned invention has been made in view of the above situations, wherein, in the embodiment thereof, a stopper is provided on the folding and bending surface of a bending punch rotatably supported on the upper wall of a die via a pin, and as the bending punch is turned downward, the free end portions are pressed to the side wall of the channel-shaped sectional portion by the stopper (that is, an axial force is applied to the bending portion), and it is described that the free end portions are folded and bent to become horizontal in this state.
However, in those prior arts described above and shown in FIG. 11 through FIG. 18, in order to evade problems such as warping, undulating, etc., of the work W15 which may occur in the process of folding or bending along its length, a structural member is welded after being folded or bent so as to become linear in its lengthwise direction. After that, bending or folding in the lengthwise direction must be carried out separately from the above processes. Therefore, the number of processes is increased, and the processes are cumbersome, wherein facilities are required to execute the processes. Such a problem arises, by which the production costs can not be decreased.
Further, in this example, adequate intervals are formed so that the end faces Wa, Wa are not buckled due to being brought into contact with each other when both side edges of the work W15 are caused to overlap on the bottom wall 60 of the channel-shaped sectional portion 51. Therefore, since intervals between both end faces Wa, Wa may vary due to a bending angle, another problem arises, by which dimensional accuracy thereof cannot be improved.
Still further, in this example, the work W15 bent and formed to become linear is welded to prevent a problem such as warping, etc., wherein since the sectional shape thereof changes due to pressing for the purpose of bending or folding in executing press-forming work when carrying out a bending or folding process in the lengthwise direction, in which, after that, such a problem arises, by which dimensional accuracy cannot be improved on the other hand, Japanese Laid-Open Patent Publication No. Sho 59-39422 of the aforementioned prior arts discloses a method for producing a box-shaped structure having a rectangular section, which is different from a structural member having a closed section formed by the present invention. And, an apparatus disclosed to produce a box-shaped structural member has a bending punch rotatably supported therein, and a stopper is provided on the folding and bending surface, wherein the construction is complex.
Also, in a production method for which the apparatus is used, the tip end faces of the free end portions are brought into contact with the stopper by turning and shifting down the bending punch, whereby the stopper functions so that the free end portions are pressed to the side wall of the channel-shaped sectional portion and are folded and bent so as to become horizontal. That is, with the aforementioned invention, the tip end faces at both sides of the free end portions of a plate-shaped material are brought into contact with the stoppers respectively installed on the folding and bending surface of the bending punch. Therefore, the free end portions may be spaced by the distance between both stoppers when the free end portions are folded and bent so as to become horizontal. Accordingly, such a problem arises as in the prior arts shown in FIG. 11 through FIG. 18, by which dimensional accuracy cannot be improved. And, in the case where the tip ends of the free end portions of both side walls of the plate-shaped member are brought into contact with each other, it is difficult to form the plate-shaped material to such a width that the tip end faces of the plate-shaped material are not brought into contact with each other, wherein the plate-shaped material is buckled since the pressing force cannot be relieved due to the side wall being planar when the tip end faces are brought into contact with each other.
And, as described in the aforementioned prior arts, where a material folded to be channel-like in advance is folded and bent by using an upper die and a lower die, and the free end portions thereof are aligned with each other during the formation, the free end portions become horizontal before the lower die and upper die are completely pressed, and if these are completely pressed to the end, the free end portions are folded inside, whereby it becomes difficult for the free end portions to be folded and bent to a predetermined shape, and it is impossible to apply the method in practical use.
The present invention was developed in view of the above problems and shortcomings, and it is therefore an object of the invention to provide a structural member having a closed section, and an apparatus and a method for producing the structural member, which can decrease production costs with a simple construction, and at the same time, can improve the dimensional accuracy.
In order to achieve the object, a structural member having a closed section according to the first aspect of the invention is characterized in that the structural member is formed of a plate-like work and has a channel-shaped sectional portion and two closed sectional portions for which both side walls of the channel-shaped sectional portion are shared, wherein the two closed sectional portions are formed so as to overlap the bottom wall of the channel-shaped sectional portion in such a state where aide edge portions of two bending pieces bent from both side walls of the channel-shaped sectional portion to the outside thereof are brought into contact with each other at the end faces thereof, each of the respective closed sectional portions is constructed as a planar portion that connects the portions adjacent to the side wall of the channel-shaped sectional portion to the side walls at a right angle, and a curved portion which is curved toward the planar portion with the end face of the side edge portion made into the top and keeps a compression stress remained in the curving direction is disposed between the planar portion and the side edge portion.
In the invention described according to the first aspect of the invention, the end faces at both side edge portions, opposed to each other, of a work are caused to overlap the bottom wall of the channel-shaped sectional portion in a state where the end faces are brought into contact with each other, and a compression stress is left at the curved portions continuous from the end faces of the side edge portions. Therefore, the shape of the closed sectional portion is kept unchanged.
In order to achieve the abovementioned object, an apparatus for producing a structural member having a closed section according to the second aspect of the invention is an apparatus for producing a structural member formed of a plate-like work and having a channel-shaped sectional portion and two closed sectional portions for which both side walls of the channel-shaped sectional portion are shared, and is characterized in that the apparatus includes a press mold formed of a punch and a die oppositely installed, which forms both side edges of the work by press forming so that both side edges are caused to overlap on the bottom wall of the channel-shaped sectional portion, a fall-down prevention means for guiding both side edges so that the end faces of the both side edges are brought into contact with each other by preventing falling of the opposed both side edges of the work to be press-formed in the press mold; the die of the press mold has a shape such that a compression stress is generated at the closed sectional portion by pressing the end faces of a work, which are brought into contact with each other by the fall-down prevention means when executing the press-forming.
In the invention according to the second aspect of the invention, when both side edges of a work is caused to overlap the bottom wall of the channel-shaped sectional portion in a state where the end faces of the channel-shaped sectional portion are brought into contact with each other, the end faces of the work can be securely opposed to each other by the fall-down prevention means, thus generating a compression stress at the closed sectional portion without fail.
In order to achieve the object, according to the third aspect of the invention, a method for producing a structural member formed of a plate-like work and having a channel-shaped sectional portion and two closed sectional portions for which both side walls of the channel-shaped sectional portion are shared is characterized in that the method includes a pre-step of forming a primary processed work equipped with the channel-shaped sectional portion and bent pieces bent from both side walls of the channel-shaped sectional portion toward the outside thereof at a right angle in the work; an intermediate step of forming a secondary processed work approximating the closed sectional portion by bending the side edges of the respective bent pieces of the primary processed work at an appointed position; and a post-step of forming the closed sectional portions by press-forming so that the end faces at both side edges, opposed to each other, of the secondary processed work are caused to overlap the bottom wall of the channel-shaped sectional portion in a state where the end faces are brought into contact with each other, and forming curved portions constituting the closed sectional portions by generating a compression stress at the work by pressing the end faces of the work, which are brought into contact with each other, to each other.
In the invention according to the third aspect of the invention, since the pre-step, intermediate step and post-step are systematically combined, it is possible to produce a structural member having a closed section with good efficiency and accuracy.
In order to achieve the abovementioned object, a method for producing a structural member having a closed section according to one preferred form of the third aspect of the invention is characterized in that, in addition to the invention as set forth in the third aspect thereof, the work is curved in the lengthwise direction thereof in the pre-step or intermediate step.
In the invention according to one preferred form of the third aspect of the invention, since the work is not bent in the lengthwise direction in the post-step, the sectional shape thereof is kept unchanged.